Most electronic devices manufactured today include one or more printed circuit boards, which may also be referred to as printed wiring boards. Such printed circuit boards typically provide support for and interconnections among electronic devices and components. A common device found on printed circuit boards is an integrated circuit, which is typically formed from a semiconductor material that is encapsulated in a ceramic or plastic package. The integrated circuit package provides input and output (I/O) pins for mechanically and electrically connecting the integrated circuit semiconductor material to conductive traces on the printed circuit board. With advances in semiconductor technology, the complexities of integrated circuit devices has increased along with improvements in the performance of the devices. In association with the increased complexity of the integrated circuit devices, larger and larger numbers of I/O pins are required.
Electrical interconnections between the integrated circuit device's I/O pins and the printed circuit board are commonly formed by soldering the I/O pins to through holes in the printed circuit board, and increasingly, to conductive pads on the surface of the printed circuit board.
Methods are known for inspecting the electrical interconnections among components, integrated circuit devices and printed circuit boards. For example, optical inspection techniques have been used, both manual and automated, to inspect the placement and connections of components and integrated circuit devices to printed circuit boards. Optical inspection techniques, however, may be incapable of adequately inspecting the interconnection between certain high density integrated circuits and printed circuit boards. For example, such techniques are generally insufficient for inspecting the increasingly common Ball-Grid-Array (BGA) type devices.
In order to inspect high-density boards and BGA-type devices, inspection systems using penetrating radiation are believed to be superior to optical inspection systems. Techniques using penetrating radiation, such as x-ray, can potentially be used to inspect electrical interconnections for a BGA-type device, including both peripheral joints and joints hidden under the integrated circuit package. In order to form an image of joints hidden under the integrated circuit package, the inspection system should be capable of generating a cross sectional horizontal image (also referred to herein as a horizontal “slice” image) at a level that cuts through the I/O pins of the device.
X-ray inspection systems that are capable of generating horizontal slice images are known. For example, U.S. Pat. Nos. 5,097,492 and 5,594,770 describe two alternative approaches to generating a cross sectional horizontal image. The device shown in U.S. Pat. No. 5,097,492 generates a horizontal cross sectional image utilizing the principles of laminography. A disadvantage of this technique, however, is that the resulting horizontal cross sectional image commonly includes blurring artifacts generated by structures outside the plane of interest. Rather than relying on the principles of laminography, U.S. Pat. No. 5,594,770 shows a system that utilizes the principles of tomography with a stationary detector. In accordance with this technique a predetermined number of off-axis transmission images may be acquired and combined to produce a horizontal slice image of a plane of interest.
The theory of tomosynthesis is well documented. Using tomosynthesis, it is possible to create the horizontal slice images through test objects at different focal planes arranged along a vertical axis. Although this technique is advantageous in seeing horizontally through the object under test and determining the quality of, for example, a solder joint, valuable information regarding the correlation among several horizontal slice images may be lost. In particular, valuable defect signature information, such as the signature of voids (air pockets) which may extend across several horizontal slices may be very diluted in a single horizontal slice image, particularly where the horizontal focal plane fails to pass through the center of the void. In case of leaded devices there may be high bridges, i.e. bridges between pins but the bridge occurs at a level higher than the fillet. Since it may not be feasible to check each and every slice, such defects may be missed.
It would therefore be desirable to have an improved method and apparatus for inspecting electrical connections.